ORGANIZED DEVELOPMENT IN PLANTS
61
a single cell, development of a more or less elaborate vegetative structure begins. Recent experimental work has been concerned with the
physicochemical aspects of the environment that control embryonic development in these plants and the role played by the maternal tissues
in determining the course of embryonic development. These studies have
called attention to the similarity of apomictic development in ferns and
mosses in which single cells or small groups of cells arise among mature
somatic cells to produce meristematic centers which form whole plants.
Sporophyte cells give rise directly to morphologically distinct gametophyte plants, and cells of gametophytic tissue give rise to sporophytes.
These problems are remarkably similar to those discussed above with
respect to embryonic development in the angiosperme and the development of "adventitious" embryos from tissues in culture. Aspects of the
problem in ferns are reviewed by De Maggio (1961) and by Bell (1959)
and in mosses by Lai (1963) and others.
In anatomical studies of apomictic development in gametophytes of
Pteridium,
Whittier (1962) was unable to show the existence of a
single-celled stage involving isolation, either physical or physiological,
from neighboring cells of the gametophyte tissue. The earliest evidence
of initiation was the presence of a small meristematic mass or center of
cell multiplication set off from adjacent cells only by differences in cell
size. Whittier pointed out that apomixis in Pteridium gametophytes was
always preceded by the development of a thickened three-dimensional
prothallus, and the embryonic structures were embedded within the
cushion-like tissue. He suggested that the inner cells had limited access
to the external environment and that this relationship might play some
role in initiating apomictic development. Such an interpretation could
be classified perhaps as a type of physiological isolation. Steward et al.
(1958) invoked the same reasoning in their early studies on carrot callus
tissue differentiation.
In the ferns, wounding of the sporophytic tissues, especially of fern
leaves, leads to increased apomictic development, in this case, the initiation of gametophyte structure directly from sporophytic cells. Death of
cells surrounding the regenerating centers has been interpreted (Wetmore
et al., 1964) as a case of physiological isolation, but also such a system
involves the release of organic substances from broken cells which could
be important in initiating new structures.
It is not the author's purpose here to review this work in detail, since
an extensive literature exists and many variables are known to influence
development in these groups. Studies with fern callus (Steeves et al.,
1955; Bristow, 1962) have also been made and relate to this subject.
Rather, the intention is to point out the similarity of the problems to
61
a single cell, development of a more or less elaborate vegetative structure begins. Recent experimental work has been concerned with the
physicochemical aspects of the environment that control embryonic development in these plants and the role played by the maternal tissues
in determining the course of embryonic development. These studies have
called attention to the similarity of apomictic development in ferns and
mosses in which single cells or small groups of cells arise among mature
somatic cells to produce meristematic centers which form whole plants.
Sporophyte cells give rise directly to morphologically distinct gametophyte plants, and cells of gametophytic tissue give rise to sporophytes.
These problems are remarkably similar to those discussed above with
respect to embryonic development in the angiosperme and the development of "adventitious" embryos from tissues in culture. Aspects of the
problem in ferns are reviewed by De Maggio (1961) and by Bell (1959)
and in mosses by Lai (1963) and others.
In anatomical studies of apomictic development in gametophytes of
Pteridium,
Whittier (1962) was unable to show the existence of a
single-celled stage involving isolation, either physical or physiological,
from neighboring cells of the gametophyte tissue. The earliest evidence
of initiation was the presence of a small meristematic mass or center of
cell multiplication set off from adjacent cells only by differences in cell
size. Whittier pointed out that apomixis in Pteridium gametophytes was
always preceded by the development of a thickened three-dimensional
prothallus, and the embryonic structures were embedded within the
cushion-like tissue. He suggested that the inner cells had limited access
to the external environment and that this relationship might play some
role in initiating apomictic development. Such an interpretation could
be classified perhaps as a type of physiological isolation. Steward et al.
(1958) invoked the same reasoning in their early studies on carrot callus
tissue differentiation.
In the ferns, wounding of the sporophytic tissues, especially of fern
leaves, leads to increased apomictic development, in this case, the initiation of gametophyte structure directly from sporophytic cells. Death of
cells surrounding the regenerating centers has been interpreted (Wetmore
et al., 1964) as a case of physiological isolation, but also such a system
involves the release of organic substances from broken cells which could
be important in initiating new structures.
It is not the author's purpose here to review this work in detail, since
an extensive literature exists and many variables are known to influence
development in these groups. Studies with fern callus (Steeves et al.,
1955; Bristow, 1962) have also been made and relate to this subject.
Rather, the intention is to point out the similarity of the problems to